1. Field of the Invention
The present invention relates to a resin sliding member and a method of manufacturing the same. In addition, the invention relates to a resin gear.
2. Description of Related Art
Due to light weight, high self-lubricating characteristics, and the like, polyamide 66 is widely used for a sliding member such as a gear of a reducer of an electric power steering apparatus or a resin bearing cage. For example, Published Japanese Translation of PCT application No. 2009-536891 (JP 2009-536891A) discloses a gear for an electric power steering apparatus which is manufactured using a resin composition containing polyamide 66; a copper thermal stabilizer; an aromatic polycarbodiimide; and an impact modifier which is EPDM rubber grafted with maleic anhydride.
On the other hand, recently, a reduction in size of an automobile component (a vehicle component) and an increase in output of a vehicle component have been required. Accordingly, the resin sliding member is required to have high mechanical strength and high stiffness. Therefore, an inorganic filler such as glass fiber is supplied to polyamide 66. For example, Japanese Patent Application Publication No. 2011-32356 (JP 2011-32356 A) discloses a resin cage for a rolling bearing which is manufactured using a resin composition containing polyamide 66 which contains glass fiber; and polycarbodiimide.
However, the cage that includes polyamide 66 containing glass fiber performs contact sliding at a high surface pressure, the wearing of the resin is likely to become severe. The reason for this is as follows. The glass fiber in the resin is peeled off during the contact sliding and functions as a hard abrasive. Thus, the resin is worn and peeled off.
In order to reduce the above-described wearing and peeling of the resin, the following methods are considered: (1) a method of appropriately treating a surface of glass fiber to improve the adhesion strength between a resin and the glass fiber; and (2) a method of optimizing the diameter and shape of glass fiber to reduce the counterpart aggressiveness of the glass fiber. In particular, (3) a method, in which the molecular weight of a resin is increased to increase the crack propagation resistance regarding a crack caused by the wearing and peeling of the resin, is effective.
However, in a case where glass fiber is kneaded with polyamide 66 whose molecular weight has been increased in advance in order to perform the method (3), there are the following problems. Due to high viscosity of the resin, stable production is difficult (torque overrun, heat generation, strand breaking, and the like occur in a kneader). Due to high viscosity of the resin, insufficient dispersion or breakage of the glass fiber occurs, and physical properties of the resin deteriorate.
Therefore, until now, there is no polyamide having a high molecular weight and containing glass fiber, and sufficient wear resistance has not yet been secured for future uses requiring a reduction in size and an increase in output.
For example, in an electric power steering apparatus, the rotation of an electric motor for steering assistance is transmitted to a steering operation mechanism after the speed of the rotation is reduced through a reducer and the output is amplified. As a result, torque assistance is provided for the steering operation mechanism which is operated by a driver. Typically, this reducer includes a metal worm as a pinion and a resin worm wheel as a rack wheel that mesh with each other. The worm wheel can be manufactured, for example, using a method including forming an annular resin member on an outer circumference of a core metal (sleeve) by injection molding (insert molding) or the like; and forming teeth on an outer circumference of the resin member by cutting or the like. The resin member is formed of a resin such as polyamide (for example, PA6, PA66, or PA46) or polyphenylene sulfide (PPS).
For example, JP2009-536891-A discloses a gear for an electric power steering apparatus which is manufactured using a resin composition containing polyamide 66; a copper thermal stabilizer; an aromatic polycarbodiimide; and an impact modifier which is EPDM rubber grafted with maleic anhydride. It is necessary that the metal sleeve portion should undergo involute spline processing to stop the rotation of the tooth portion. This causes an increase in costs. In addition, recently, a reduction in environmental burden has been required. Therefore, a reduction in the weight of a vehicle component has been required, and thus, the weight of a reducer of an electric power steering apparatus also needs to be reduced. The sleeve portion of the worm wheel is formed of metal and accounts for a large proportion in the total weight of the electric power steering apparatus. Therefore, it is necessary to use a lighter material for forming the sleeve portion, while ensuring necessary mechanical strength and stiffness.
On the other hand, recently, an attempt to form a sleeve portion using a resin to reduce the weight thereof has been made (for example, refer to US 2007/0087617 A). Here, polyamide provided with glass fiber is used in a sleeve portion in order to impart mechanical strength, stiffness, and dimensional stability to the sleeve portion. On the other hand, non-reinforced high-molecular-weight polyamide (containing no glass fiber) having excellent toughness is used in a tooth portion to impart wear resistance and creep resistance to the tooth portion. The tooth portion is formed by inserting a resin sleeve portion into a mold of a molding machine and injecting a resin to an outer circumference of the sleeve portion.
There is a difference in linear expansion coefficient between the resin constituting the sleeve portion and the resin constituting the tooth portion. Therefore, due to a difference in expansion and shrinkage rate at a high temperature or a low temperature, impact resistance of a product is poor, which may cause heat shock cracking or the like. Due to expansion, a gap may be generated between the sleeve portion and the tooth portion, which may cause a decrease in durability life. Further, since it is necessary to form the sleeve portion and the tooth portion through separate injection molding processes, the manufacturing costs are high.
As a countermeasure against the above-described problems, a method of using the same glass fiber reinforced resin in both a sleeve portion and a tooth portion to integrally mold the tooth portion and the sleeve portion is considered. In this case, however, it is considered that the wearing of the tooth portion becomes severe, which results in a decrease in durability life. The reason for this is as follows. The glass fiber in the tooth portion is peeled off during the contact sliding and functions as a hard abrasive. Thus, the resin constituting the tooth portion is worn and peeled off.
On the other hand, a method of increasing the molecular weight of a resin to suppress the wearing and peeling of the resin caused by glass fiber may be proposed, but there is a limit in increasing the molecular weight through polymerization of monomers. In addition, from the viewpoint of productivity, it is difficult to knead glass fiber with a resin whose molecular weight has been increased in advance.